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Participant

Join Date: Jun 2007
Posts: 2

Control Valve Mechanical Stops

06/21/2007 3:35 PM

Can someone point me to a good description (with pictures) of a mechanical stop installed to limit travel of a control valve? I'm designing a safety system and need to ensure minimum flow through a control valve.

Thanks,

Duane

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Guru
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#1

Re: Control Valve Mechanical Stops

06/21/2007 3:55 PM

I don't know the size and type of your valve but one possibility would be to drill a hole through the sluice. you would have to install a normal valve next to it to obtain shutoff when required.

Another possibility is to install a small valve in parallel to the bigger one and remove the handwheel.

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Participant

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#2
In reply to #1

Re: Control Valve Mechanical Stops

06/21/2007 4:01 PM

A drilled hole would probalby work for a smaller valve. These are 8" and 10" control valves. I recall a clamp arrangement on the shaft that limits travel but can't find a reference in literature.

Thanks,

Duane

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#3

Re: Control Valve Mechanical Stops

06/21/2007 9:06 PM

If it is a control valve with an actuator look in the actuator literature the stop may be built in. Depends on the type diaphram or cylinder type. go to www.fisher.com for all the pictures and valves.

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#4

Re: Control Valve Mechanical Stops

06/22/2007 4:03 AM

If a minimum flow is required, one needs to make allowance for the failure of the energy supplies to the control valve and the failure of the device(s) providing the required minimum flow. Some thoughts:

  • It might be more appropriate to install a fixed restriction as a bypass to the control valve, thereby ensuring a minimum flow in the event of the control valve being shut.
  • It is necessary to allow for the possibility of the minimum flow not being achieved despite the control valve being wide open, because of the failure of something else, like a pump.
  • It might be necessary for the valve to "fail open".
  • It might be possible to assemble the valve in such a way that the mechanism at full travel never quite closes the valve, avoiding the need for an add-on mechanical stop.
  • It might be possible to configure the control system in such a way that there is always a signal to the valve that is different to that required to provide the shut position.

Assuming, perhaps incorrectly, that the poster has a mechanical specialism then some dialogue with a colleague that specialises more in control system design on some of these points is highly recommended, as some of the above might be easier to provide and maintain than what is being suggested.

A formal review and record of decisions, such as the methodology embraced within a Hazop Study, is also highly recommended.

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#5

Re: Control Valve Mechanical Stops

06/22/2007 4:35 AM

One way is to install a clamp on the valve stem and limit the movement of the stem.

OR calibrate the valve to a higher range than the signal ( positioner output is 3-15 psi,whereas the valve is calibrated for 3- 30 psig).

Result:the valve will open approximately 50% when the signal is for full opening (15 psig).

we can actually work out the required opening and the exact signal pressure required .

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#6

Re: Control Valve Mechanical Stops

06/22/2007 1:08 PM

Hi:

I used to work for a control valve manufacturer. We solved this problem for control valves which are equipped with air diaphragm actuators, normally closed, air-to-open. These actuators can be equipped or modified to accept hand operating devices, HOD's we called them. The HOD functions by pulling up the diapragm plate in the air chamber of the actuator. The HOD did not interfere with normal valve stroking. It's only function was to provide a way to open the valve on loss of air. But, what we did was to open the valve with the HOD to limit the amount of closure. So, we just set the valve up to be open a small amount when the air signal said to close. The valve would close until it hit the pull open stop and sit there. So, the valve would stroke normally, springs didn't need to be readjusted, stroke length didn't need to be changed and this will work with or without a positioner-fitted valve. In addition, you can "tune" the amount of opening during initial start-up, remove the handwheel and stick a length of pvc pipe with a cap on the end so no one would get impaled. In addition, you didn't lose the ability to open the valve on loss of air. Most DCV manufactureres have kits to convert their actuators.

Good luck, drehmeyer.

Hank

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Commentator

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#7

Re: Control Valve Mechanical Stops

06/23/2007 11:44 AM

Hi,

Are you controlling the flowrate of a liquid using a control valve? do you need accurate flowrate? if not, then pressure reducing valve will work. if it is something precise, then you need an expensive control system. a pressure reducing valve has its minimum and maximum flowrate/pressure rating. however, you cannot shut-off. you need to install independent isolation to shut-off your line.

regards

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Member

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Posts: 9
#8

Re: Control Valve Mechanical Stops

06/24/2007 10:43 AM

This reminds me of the old story of a mathematician, a physicist and a mechanical engineer who were asked to calculate the volume of a red rubber ball.

The mathematician measured the ball and calculated the volume. Because the ball was not a perfect sphere and the calipers tended to press into the ball, the mathematician never got the same answer twice.

The physicist submerged the ball in a graduated beaker full of water to measure the displacement but because the ball wanted float, he had to keep pushing the ball down with a stick and would sometimes be measuring the volume of that as well. He had to average his results with a plus or minus tolerance.

The mechanical engineer looked up the model and serial number in the vendor's Red Rubber Ball Catalog and referenced the volume stated by the manufacturer.

Ask Hankt whom he used to work for and give them a call.

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Users who posted comments:

510inspector (1); drehmeyer@bakerrisk.com (1); Hankt (1); Hendrik (1); JasBond (1); jojie_oak (1); NatNat (1); PWSlack (1)

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